1. Field of the Invention
The present invention relates generally to methods of determining subterranean formation characteristics, and more particularly, to methods of determining stimulation characteristics of a subterranean formation prior to performing a stimulation treatment therein.
2. Description of the Prior Art
A variety of stimulation procedures have heretofore been developed and used for increasing the production of oil and/or gas from subterranean producing formations. The most common of such procedures involves the creation and propping of fractures in the formation to provide flow passages through which oil and gas can flow from the formation to the well bore, and/or acidizing the formation to increase the permeability thereof or to etch flow channels in the faces of created or natural fractures in the formation. A particular stimulation procedure is selected based on the stimulation characteristics of a subterranean formation or zone to be stimulated, i.e., the type of rock or other material making up the formation, the porosity of the formation, the permeability of the formation, the abundance of natural fractures in the formation, the presence or absence of high permeability streaks or zones in the formation, the rate of fluid loss into the formation while the stimulation procedure is carried out and whether or not the formation permeability is damaged when particular stimulation fluids are utilized. While some of the foregoing characteristics are generally always known about a particular formation or zone to be stimulated, very often a number of important characteristics are not known. As a result, performances of subterranean formation production stimulation treatments are not always successful in increasing the production of hydrocarbons, and sometimes permanent damage in the form of lost permeability and lower production results.
A commonly used technique for determining the characteristics of subterranean formation materials involves the taking of individual core samples from the subterranean formation and testing the samples to determine one or more characteristics of the type described above. The heretofore utilized testing techniques have generally been time consuming, individual samples may be biased due to spatial heterogeneity of the formation and have not provided all of the important characteristics required for determining if a subterranean formation can be stimulated; and if so, how best to accomplish such stimulation.
Recently, X-ray computed tomography (CT) technology has been applied to the determination of subterranean formation properties. Computed tomographic scanning instruments can produce a cross-sectional image of a core sample along any chosen axis. The plane of the image can be moved in discrete intervals to obtain information in three dimensions. Such X-ray computed tomographic scanning has been utilized to determine information useful in enhanced oil recovery and fluid mobility control in subterranean formations. For example, the permeabilities of core samples of formation materials to various fluids at varying conditions of temperature and pressure can be determined. By subtracting the images of the core samples before and after saturation with a particular fluid, the distribution and penetration of the fluid can be determined. The CT scanning technique is non-destructive and can, for example, be conducted while the core sample is contained in a pressurized or other core preservation container.
U.S. Pat. No. 4,649,483 issued Mar. 10, 1987 describes a method of determining the oil, gas and brine content of core samples taken from subterranean formations utilizing computed tomographic scanning. The sample is scanned with X-rays of differing energies in the fluid saturated and fluid extracted states. The computed tomographic images produced are utilized in the determination of the X-ray mass attenuation coefficients for the sample and the extracted fluid. From the mass attenuation coefficients, the weight fractions and volume fractions of each of the extracted fluids are determined.
U.S. Pat. No. 4,688,238 issued Aug. 18, 1987 discloses a method of using computed tomographic scanning on a core sample to determine pore volume changes, pore compressibility and core fracturing over a range of pressures.
U.S. Pat. No. 4,799,382 issued Jan. 24, 1989 discloses methods for determining reservoir characteristics of a subterranean formation core sample. The core sample is subjected to pressure cycling and scanned with X-rays at least once each pressure cycle. Computed tomographic images of the sample for each pressure cycle are produced, and core sample fracturing is determined from the produced images.
While the above described formation core sample testing methods using computed tomography scanning have been useful in determining the subterranean formation properties specified, various formation characteristics useful in determining if a subterranean formation can be stimulated and how best to accomplish such stimulation have not been determined.
Thus, there is a need for improved methods of determining stimulation related characteristics of subterranean formations or zones prior to stimulating the formations or zones.